Fast reactors, and fast breeder reactors in particular, generally consist of a central core of fissile material surrounded by a fertile material blanket. In one design, the core is composed of a number of subassemblies each containing a multiplicity of fuel pins or elements. Each fuel pin is a closed cylinder and contains a column of fuel material, typically mixed uranium and plutonium oxide, with columns of fertile material, typically uranium oxide, located above and below the fuel column to form axial blankets. Fission products produced as a result of the fission of fissile and fertile materials are retained in the fuel pin during reactor operation.
Cesium is one of the most abundant of the fission products produced during the fission process. With a low melting temperature and a high vapor pressure at typical oxide fuel operating temperatures in liquid metal fast breeder reactors, cesium readily relocates both radially and axially within a fuel pin. As a result of the cesium migration, cesium will tend to accumulate in highly localized areas in the fuel pin. These localized cesium accumulations are often associated with localized cladding strain, fuel deformation, and fragmentation of fuel and blanket pellets. In some cases, localized cesium concentration is believed to have been responsible for premature cladding failure.